X-rays and gamma rays have long been used to determine qualities such as the mass density and the thickness of materials. In particular applications, these radiation have been used to examine hydrocarbon-carrying pipelines to inspect them for wear and corrosion. In spite of the applied protective coating, pipelines are nevertheless subjected to corrosion at the surface. Because of the highly corrosive nature of crude oil, crude-carrying pipelines are also subjected to erosion at the inner surface. For safety reasons, periodic inspection of pipelines for mechanical and physical integrity is mandatory. Desirable inspection procedure includes the detection of the presence and the amount of moisture and rust at the surface, and material losses on the inner wall. Pipelines are generally covered with a thick (4 inch) insulator for the purpose of heat retention or protection against corrosive components in the atmosphere. Removal of the insulator is expensive and, if asbestos is incorporated in the insulator, hazardous. Inspection should therefore be carried out with the insulator in place, without compromising speed, cost, and reliability.
Many X-ray techniques have been proposed or are in use for the purpose of measuring the wall thickness of pipelines. These techniques can be divided into two broad categories: "transmission" and "backscatter" techniques.
The "transmission technique" is similar to the method used by a medical X-ray machine. In the transmission technique, an object, such as pipeline, is bombarded with radiation. The radiation that is transmitted through the object is received on a film or similar detector. The radiation transmitted reveals information about the object. For example, a photograph qualitatively depicting pitting within a pipe can be generated using the transmission technique.
The transmission technique is unsatisfactory for most pipeline inspection needs for several reasons. First, pipeline often runs in bundles, so that it is not possible to detect transmitted radiation on a particular stand of pipe. Second, the transmission technique is inaccurate because both sides of the pipe are unavoidably measured at the same time resulting in a sum. Third, the measurement requires high radiation density and high tube voltage and the method detects only localized qualitative deviations, such as pitting, within the area of the applied incident radiation beam. In practice, the transmission image is recorded on film and assessments are subjectively made by trained technicians. Thus, the transmission technique is essentially useless in quantitatively determining precise wear absent expensive, indepth image analysis.
The "backscatter" technique uses a scanning detector or detector array to form a line image in the depth dimension of the steel pipe as it is penetrated by a fine X-ray beam. The scatter intensity is proportional to the local electron density of the sample and to the X-ray intensity at a specific point. Ideally, the resulting image is a density profile of the sample along the incident beam and can reveal the presence of moisture, steel, and corrosion. From the density transitions, with rust on one side and the hydrocarbon products on the other side, the thickness of the uncorroded steel may be found. Thickness determination based on density changes works well with low-absorbing material like plastics and aluminum where the envelope of the image does not decay too rapidly with depth. In steel pipes, serious practical problems arise because the steel/crude boundary becomes blurred and cannot be distinguished from the rapidly decaying intensity. This blurring is due to the point spread function of the system and the rapid decay function of the steel.
To sharpen the blurred backscatter image caused by rapidly decaying scatter intensity, the area of the incident radiation beam as well as the detector collimator width need to be reduced, both resulting in a reduction in intensity. In fact, to be useful for pipeline applications, the diameter of the incident radiation beam must be much smaller than the thickness of the steel pipe. For example, if losses of 10% in wall thickness are to be detected, the incident radiation beam must be at least 10% of that dimension. An increase in voltage of the X-ray source may be used to improve the total intensity and reduce the absorption in steel, but such an approach is impractical. First, sources of sufficiency high voltage are not readily available commercially. Second, the increase in background noise and equipment weight make using such a radiation source impractical. Third, such a radiation source would increase the danger of radiation exposure to those in the vicinity of the source. Those skilled in the art have directed their efforts at developing imaging techniques to reduce blurring caused by the point spread function.